阅读说明：本技术 对每个开关功能用并联模块的人工稳定短路故障模式功能 (Artificially stabilized short circuit fault mode functionality for each switch function with parallel modules ) 是由 J·施泰因克 P·梅巴克 于 2019-09-05 设计创作，主要内容包括：本公开的实施例涉及对每个开关功能用并联模块的人工稳定短路故障模式功能。本申请公开了一种用于电压源变换器单元的开关装置,该电压源变换器单元具有两个AC端子,其中开关装置形成多个并联的串联电路(分支、电流路径),其中串联电路路径中的每个串联电路路径中的开关由外部信号控制,以在“接通”状态和“关断”状态之间改变开关的导电状态。外部信号由一个或多个控制单元生成,并且两个AC端子各自连接到开关的串联电路的子集。控制单元包括故障检测装置,故障检测装置适于通过预定条件确定串联电路中的有缺陷的开关。控制单元还被配置为：如果根据预定条件确定开关中的一个开关是有缺陷的,则输出控制信号以改变串联电路中的开关中的每个开关的导电状态,使得两个AC端子之间的短路被创建。(Embodiments of the present disclosure relate to artificially stabilizing short circuit fault mode functionality with parallel modules for each switching function. The present application discloses a switching arrangement for a voltage source converter cell having two AC terminals, wherein the switching arrangement forms a plurality of parallel series circuits (branches, current paths), wherein the switches in each of the series circuit paths are controlled by an external signal to change the conducting state of the switches between an "on" state and an "off" state. The external signal is generated by one or more control units and the two AC terminals are each connected to a subset of the series circuit of switches. The control unit comprises fault detection means adapted to determine a defective switch in the series circuit by means of a predetermined condition. The control unit is further configured to: if it is determined that one of the switches is defective according to a predetermined condition, a control signal is output to change the conduction state of each of the switches in the series circuit such that a short circuit between the two AC terminals is created.)

1. A switch (101, 102, 111, 112) arrangement for a voltage converter unit having two AC terminals (AC1, AC2), wherein

The switch (101, 102, 111, 112) arrangement forms N +1 parallel current paths (100, 110), wherein N is greater than or equal to '1', and wherein

The switch (101, 102, 111, 112) in each of the N +1 current paths (100, 110) is controlled by an external signal to change the conductive state of the switch (101, 102, 111, 112) between an "on" state and an "off" state,

the external signals are generated by one or more control units (160), and wherein:

the two AC terminals (AC1, AC2) are connected to the N +1 current paths (100, 110) with the switches (101, 102, 111, 112), wherein

The control unit (160, 170) comprises a fault detection means adapted to determine a defective switch by a predetermined condition, and wherein

The control unit (160) is further configured to: outputting a control signal to change the conductive state of each of the N +1 current paths (100, 110) such that a short circuit between the two AC terminals (AC1, AC2) is created if one of the switches (101, 102, 111, 112) is determined to be defective according to the predetermined condition, and wherein

A semiconductor-based shorting device (141) forms a current path (140) that is switched electrically in parallel with the N +1 current paths, and is configured to: in case the switches (101, 102, 111, 112) are determined to be defective, the DC-link capacitor (150) is discharged and the AC terminals are permanently short-circuited.

2. The switch (101, 102, 111, 112, 121, 122, 131, 132) arrangement for a converter cell according to claim 1, wherein the switch is a semiconductor.

3. The switch (101, 102, 111, 112, 121, 122, 131, 132) arrangement for a converter cell according to any of claims 1 and 2, wherein the semiconductor is an IGBT.

4. The switching arrangement according to any of the preceding claims, wherein the switch (141) comprises a semiconductor-based short-circuiting device.

5. The switching device according to any one of the preceding claims, wherein a capacitor (150) is connected electrically in parallel with the N +1 current paths.

6. A modular multilevel converter cell comprising a switch (101, 102, 111, 112, 121, 122, 131, 132, 141) arrangement according to any of claims 1 to 5, wherein the switch arrangement is adapted for a modular multilevel converter cell having a half-bridge topology or for a modular multilevel converter cell having a full-bridge topology.

7. The modular multilevel converter cell of claim 6, further comprising a network interface for connecting the converter cell to a data network, wherein the converter cell or a controller of the converter cell is operatively connected to the network interface for at least one of executing commands received from the data network and transmitting device status information to the data network, and wherein the network interface is configured to transceive digital signals/data between the converter cell and the data network, wherein

The digital signals/data include operational commands and/or information related to the device or the network.

8. A method for shorting AC terminals of a modular multilevel cell, comprising

Determining defective switches (101, 102, 111, 112, 121, 122, 131, 132) by a predetermined condition;

activating a current path (140) in the form of a semiconductor-based short-circuiting device (141) or activating switches (101, 102, 111, 112, 121, 122, 131, 132) in a plurality of current paths (110, 100) such that a short-circuit between the AC terminals is generated.

9. The method for shorting AC terminals according to claim 8, comprising:

activating the switches (101, 102, 111, 112, 121, 122, 131, 132) in such a way that: the switch, which is considered healthy according to the predetermined condition and arranged in parallel with the switch determined to be defective, is set to a permanent "on" state together with a further switch connected to the same DC potential as the failed switch, or

Activating the semiconductor-based shorting device (141), forming a current path (140) that is switched electrically in parallel with the plurality of current paths to discharge the DC link capacitor (150) and permanently short the AC terminal in the event that a switch (101, 102, 111, 112) has failed.

Technical Field

The present application relates to the field of voltage conversion systems in the medium or high voltage region. In particular, the present application relates to the field of construction and operation of Modular Multilevel Converter (MMC) cells. The present application provides methods and devices for improved fault tolerance of MMC cells in the event of high power semiconductor failure.

Background

A semiconductor module usually comprises several parallel chips according to a switching function, each chip being connected to an associated electrode via a bonding wire. A typical failure mode of such a module is that one of the chips arranged in parallel fails and may short the module terminals. This may also include control terminals, such as the gate and emitter of an IGBT.

Thus, healthy chips in a parallel arrangement are no longer controlled by standard gate drivers. In converters like two-level voltage source converters this may not be problematic as the switching function may be unique and the converter must be shut down anyway once it is no longer possible to switch between on and off as required.

But in converters called chain-link converters or Modular Multilevel Converters (MMC), the modules are providing switching functions for cells (full bridge cells for chain-link converters or half bridge cells for modular multilevel converters).

These converters include cells connected in series and it is desirable that they do not stop operating after a single semiconductor fault. This may be achieved by shorting the AC terminals of the associated cells. Since modules comprising damaged chips cannot be loaded continuously and safely with operating current, module-based chain links and MMCs comprise dedicated AC terminal shorting devices attached between the AC terminals. These may be mechanical switches, moved by springs, electromechanical or thermo-mechanical forces or even additional semiconductor switches. These switches or devices add cost without contributing any additional operational advantages.

There are several ways to at least ameliorate the overload condition of the switch. One is to replace the module with one having a higher power capacity. But increasing the allowable operating current in the converter by replacing smaller modules with larger modules may not improve the situation in a fault condition. Therefore, a solution for improved control of power modules is highly desirable.

Disclosure of Invention

To address the above and other potential problems, in a first aspect, embodiments of the present disclosure propose a switching device for a voltage converter cell.

The voltage converter unit may have two AC terminals. The switching means may form 2N parallel current paths. As an alternative to the current path, the terms "branch" or "series circuit" or "series connection" may alternatively be used. The number N may be greater than or equal to "2", and wherein the switch in each of the 2N series connections is controlled by an external signal to change the conductive state of the switch between an "on" state and an "off" state. The external signal may be generated by one or more control units and the two AC terminals may each be connected with a subset of the 2N current paths through the switches. The control unit may comprise fault detection means, which may be adapted to determine a defective switch by means of predetermined conditions. The control unit may be further configured to: if it can be determined that one of the switches is defective according to a predetermined condition, a control signal is output to change the conduction state of each of the 2N current paths so that a short circuit between the two AC terminals can be created.

In another aspect, a switching device for a voltage converter unit having two AC terminals may be disclosed. The switching means may form N +1, two parallel current paths or branches. The number N may be greater than or equal to "1", and wherein the switches in each of the N +1 current paths are controlled by an external signal to change the conductive state of the switches between an "on" state and an "off state, the external signal being generated by one or more control units, and wherein the two AC terminals are connected to the N +1 current paths through the switches, wherein the control units comprise fault detection means adapted to determine a defective switch by a predetermined condition, and wherein the control units are further configured to: if it can be determined that one of the switches is defective according to a predetermined condition, a control signal is output to change the conductive state of each of the N +1 current paths/branches/series connections so that a short circuit between the two AC terminals can be created.

In yet another aspect, a modular multilevel converter cell may be disclosed. The modular multilevel converter cell may comprise a switching arrangement according to an aspect or embodiment of the invention, wherein the switching arrangement may be adapted for a modular multilevel converter cell having a half bridge topology or for a modular multilevel converter cell having a full bridge topology.

Another aspect discloses a method for shorting AC terminals of a modular multilevel converter cell. The method may particularly comprise determining a defective switch by a predetermined condition and particularly activating the current path formed by the semiconductor-based short-circuiting device or activating the switch in the current path such that a short circuit between the AC terminals may be generated.

Drawings

Embodiments of the present disclosure will be presented by way of example and their advantages explained in more detail below with reference to the accompanying drawings, in which:

fig. 1 shows an example of a full bridge cell according to an embodiment of the present disclosure, wherein a lightning symbol indicates a faulty semiconductor switch as an example;

fig. 2 shows an example of a half bridge cell according to an embodiment of the present disclosure, wherein a lightning symbol indicates a faulty semiconductor switch as an example;

fig. 3 shows an example of a half-bridge cell with a crowbar, wherein a lightning symbol indicates a faulty semiconductor switch as an example, according to an embodiment of the present disclosure;

fig. 4 shows an example of a full bridge cell with a crowbar, wherein a lightning symbol indicates a faulty semiconductor switch as an example, according to an embodiment of the present disclosure;

FIG. 5 shows half-bridges and full-bridges without crowbars currently in use;

fig. 6a shows a part of a converter phase link with series connected chain-link cells (full bridge) without crowbar and without parallel switch modules;

fig. 6b shows a part of a converter phase link with series connected chain link cells (full bridge) without crowbar/parallel switch modules but with an AC short-circuit device.

Detailed Description

Hereinafter, the principle and spirit of the present disclosure will be described with reference to illustrative embodiments. It is understood that all of these examples are given only to those skilled in the art to better understand and further practice the present disclosure, and are not intended to limit the scope of the present disclosure. For instance, features illustrated or described as part of one embodiment, can be used with another embodiment to yield a still further embodiment. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions should be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

The disclosed subject matter will now be described with reference to the drawings. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the detailed description with details that are well known to those skilled in the art. However, the attached drawings are included to describe and explain illustrative examples of the disclosed subject matter. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.

In the following, the term "current path" is in particular a series connection or a series circuit of two switches. In particular, this refers to a series circuit of two semiconductors. The semiconductor may preferably be any kind of transistor that can meet the technical requirements of the converter cell. Known transistor types are for example IGBT or SiC transistors.

The junction (where the emitter of the first transistor may be connected to the collector of the second transistor or the drain of the first transistor may be connected to the source of the second transistor) may be referred to as a "mid-point connection" or "intermediate connection".

An improvement to the chain-link converter can be achieved if the control channels of the parallel modules can be operated in such a way that a short circuit of the output of one channel does not disturb the operation of the remaining channels.

In case of a failure of a chip in one of the modules, the parallel "healthy" switch (meaning the switch that can still be activated or deactivated by its driver GD) may be permanently turned on together with another H-bridge switch connected to the same DC link potential as the failed switch.

Thus, the AC terminal is short circuited ("zero vector"), while the switch connected to the other terminal of the DC link remains off and blocks the DC link voltage. By this, the installation of additional AC short-circuiting devices can be avoided. When using a healthy switch in combination with a DC short-circuiting device, the extra costs and space required for an AC short-circuiting device can be avoided by having at least one parallel path in each current path provided. Fig. 6b shows an example of a power converter phase leg with series connected chain-link cells. Between the AC1 terminal and the AC2 terminal, an AC short device may be arranged.

The full-bridge topologies in fig. 1 and 4 show an example with two parallel semiconductor devices per switching function. Semiconductor T3a of 121 in path 120 may suffer from a short circuit. The full-bridge cell in fig. 1 and 4 may no longer be operated. In large chain-link converters, it may be desirable to keep the converter operational. This means that a short circuit between the AC1 and AC2 connections must be established.

The total output voltage form of the converter can be slightly deformed by the absence of cells, but the converter is still operable, as the number of cells in such a chain-link converter can be very high, and the absence (but short-circuiting) does not render the converter unusable. To achieve converter operability and deactivate the unit, the semiconductor T1a (and preferably T1b) in branches 100 and 110 of fig. 1 and 4 is activated from driver GD 1. T3a may be assumed to have a "short" (currently "on"). Current between AC1 and AC2 may flow in both directions due to "freewheeling" diodes arranged in parallel with the semiconductors (transistors).

Depending on the actual converter design, the current carrying capacity of a chain-link or MMC cell may be increased with some or all of the capacity of the modules added to provide the parallel path. The minimal advantage achieved may be the ability to increase the short term load capacity of the unit with the additional capacity of the modules added to provide parallel paths. That is, the extra cost of the additional current path is rewarded by the extra performance.

Adding a DC crowbar (which will become a permanent short circuit once the DC capacitor energy can be partially discharged via the DC crowbar) to the cells with parallel current paths allows a stable AC short of the half-bridge and full-bridge cells without active control of the switches 101, 102, 111, 112, 121, 122, 131, 132 of the cells. Fig. 3 (half bridge with two parallel branches 100, 110 controlled by the same gate driver 160 (GD)) and fig. 4 (full bridge with two parallel branches in each channel) show this embodiment using an additional crowbar 141, which crowbar 141 can provide a stable short circuit between AC1 and AC 2. The freewheeling diode enables current to flow in both directions.

Thus, in the first embodiment, an arrangement of switches 101, 102, 111, 112, 121, 122, 131, 132 for a voltage converter unit may be disclosed. The voltage converter cell may have two AC terminals AC1 and AC 2. The AC terminals are configured to output an AC current converted from a DC voltage (connected at DC + and DC-, see the figure) by the voltage converter unit. The arrangement of switches 101, 102, 111, 112, 121, 122, 131, 132 forms 2N parallel current paths or branches 100, 110, 120, 130.

For a full-bridge arrangement of switches, the total number of parallel branches/ current paths 100, 110, 120, 130 used (see fig. 1 and 4) may preferably be an even number. The number N may be greater than or equal to "2".

The switches 101, 102, 111, 112, 121, 122, 131, 132 in each of the 2N current paths 100, 110, 120, 130 are controlled by signals to change the conductive state of the switches 101, 102, 111, 112, 121, 122, 131, 132 between an "on" state and an "off" state, wherein the signals are generated by one or more control units 160, 170. The control unit 160, 170 controlling the switches may preferably be within an MMC cell.

The two AC terminals (AC1, AC2) are each connected with a subset of the 2N current paths 100, 110, 120, 130 having switches 101, 102, 111, 112, 121, 122, 131, 132, wherein the control unit 160, 170 comprises fault detection means adapted to determine a defective switch by a predetermined condition, and wherein the control unit 160, 170 is further configured to output a control signal to change the conductive state of each of the 2N current paths 100, 110, 120, 130, such that a short circuit loop between the two AC terminals (AC1, AC2) may be created, if one of the switches 101, 102, 111, 112, 121, 122, 131, 132 may be determined to be defective according to the predetermined condition.

The "on" state of a semiconductor (transistor) may represent the state when the semiconductor has the lowest possible resistance (data sheet). The "off state represents the semiconductor state with the highest possible resistance. Values between these maximum/minimum values can render the semiconductor defective, since if the switching process between "on" and "off takes too long, depending on the power that P ═ R × I2 would generate in the semiconductor, it would immediately overheat and destroy the semiconductor.

The subset of the current path or "branch" using another expression may preferably be an even number. A subset of the full bridges in fig. 1 and 4 have two current paths 100, 110, 120, 130 or branches. Each subset may be connected to a control unit 160 or 170.

In another aspect, an apparatus for a switch (101, 102, 111, 112) of a voltage converter unit may be disclosed. Voltage converter unit having two AC terminals (AC1, AC2), wherein the arrangement of switches 101, 102, 111, 112 forms an N +1 number of two parallel current paths 100, 110, wherein the number N may be equal to or greater than "1", and wherein the switches 101, 102, 111, 112 in each of the N +1 current paths 100, 110 are controlled by a signal to change the conductive state of the switches 101, 102, 111, 112 between an "on" state and an "off" state. The signals may be generated by one or more control units 160. The control unit 160, 170 controlling the switches may preferably be within an MMC cell. The two AC terminals AC1, AC2 may be connected to the N +1 current paths 100, 110 with switches 101, 102, 111, 112. The control unit 160 may comprise fault detection means adapted to determine a defective switch (semiconductor/transistor) by a predetermined condition, and wherein the control unit 160 is further configured to output a control signal to change the conductive state of each of the N +1 current paths 100, 110 such that a short circuit between the two AC terminals (AC1, AC2) may be created, if one of the switches 101, 102, 111, 112 may be determined to be defective according to the predetermined condition.

Arrangement of switches 101, 102, 111, 112, 121, 122, 131, 132 for a converter cell according to any of claims 1 or 2, wherein the switches comprise semiconductors.

According to an embodiment of the application, the semiconductor may preferably be a transistor for the arrangement of switches 101, 102, 111, 112, 121, 122, 131, 132 of the converter cell. In particular, the transistor may preferably be an IGBT (insulated gate bipolar transistor), which may be arranged in a specific module. Other types of transistors (e.g., SiC MOS FETs) are possible depending on their current and voltage ratings.

Arrangement of switches 101, 102, 111, 112, 121, 122, 131, 132 for a converter cell according to any of the preceding claims, wherein the switch 141 may form a current path 140. The current path or branch 140 may preferably be switched electrically in parallel with 2N current paths or N +1 current paths.

For MMC cells (half-bridges), an active short circuit that damages the switch does not contribute to all failure modes. Therefore, it cannot be applied as a general solution. The half-bridge cell has one AC terminal of a DC capacitor connected to it (see fig. 2). In case of failure of a switch not connected between the two terminals, a DC voltage may be continuously applied between the AC terminals.

Thus, shorting the AC terminals at the same time shorts the DC capacitor 150. In many cases this will lead to an explosion of the semiconductors 101, 102, 111, 112, 121, 122, 131, 132 (IGBTs), which may be in the path of the discharge current. The energy is too high to dissipate in the transistor path. In particular, the semiconductors 101, 102, 111, 112, 121, 122, 131, 132 are not able to instantaneously dissipate the energy that may be stored in the capacitor 150. In practice, the current path or branch 140 may be placed as a DC capacitor discharge device (also referred to as a "crowbar") between the DC capacitor terminals.

By activating (triggering) the semiconductor (also referred to as a "crowbar"), the capacitor 150 can be discharged in a dedicated path designed for this event. In this case, it may be avoided that a decoupled control channel for the parallel modules has to be used if the crowbar provides a continuous short circuit. In this case, the freewheeling diode provides a short-circuit path for both current directions for one current direction through the crowbar. This function can be ensured only in the case of parallel modules, since the on-state of the diode chips in the module with damaged chips cannot be ensured.

The provision of a full bridge cell (with a DC crowbar, see fig. 4) consisting of at least two parallel modules in each channel also allows to permanently short the AC terminals without the need to be able to actively control the state of the semiconductors.

The freewheeling diode arranged in parallel between the collector/emitter (or drain/source) contacts of the transistors in the switches in the full bridge, together with a DC crowbar that may permanently short the DC capacitor, may advantageously be able to permanently short the AC terminals without any active control of The Semiconductors (IGBTs).

In another embodiment of the present application, the switch 141 forming the branch 140 comprises a semiconductor-based shorting device, according to other embodiments. Preferably, the semiconductor for the short-circuiting device may be a thyristor. Any other semiconductor capable of providing such a short circuit function for an MMC cell may be used. The semiconductor is not limited to thyristors.

In another embodiment, in the switching device according to any of the embodiments, the capacitor 150 may be electrically connected to 2N current paths or N +1 current paths in parallel. The capacitor can be considered a DC capacitor that stores energy for the cell.

In a further embodiment, in the switching device according to any of the embodiments, each of the 2N or N +1 current paths or branches 100, 110, 120, 130 may comprise a series connection of two switches 101, 102, 111, 112, 121, 122, 131, 132. Each series connection of the two switches 101, 102, 111, 112, 121, 122, 131, 132 has an electrical connection point at this position, wherein the two switches of the current path are connected. Preferably, the connection point or "midpoint" may represent a position in which the source and emitter of the transistor are connected. Midpoints in each subset of the 2N branches (full-bridge arrangement) are electrically connected and form AC1 and AC2 terminals. Preferably, for a full bridge, the number of subsets may be "2". Similarly, the midpoints in the half-bridge arrangement (N +1 branches) are electrically connected.

In another embodiment, in the switching device according to the embodiment, the connection point of the series connection of the two switches 101, 102, 111, 112, 121, 122, 131, 132 in each subset of the 2N current paths is electrically connected to one of the AC terminals AC1, AC 2.

In another embodiment, in the switching device according to the embodiment, the connection point of the series connection of the two switches 101, 102, 111, 112, 121, 122, 131, 132 of the N +1 current paths may be connected to a first one of the AC terminals AC1, AC2, and a second one of the AC terminals AC1, AC2 may be connected to the "-" potential of the DC line.

In another aspect of the present disclosure, a modular multilevel converter cell of an apparatus comprising switches 101, 102, 111, 112, 121, 122, 131, 132, 141 according to an embodiment of the present invention may be disclosed. The switching arrangement may be adapted for a modular multilevel converter cell having a half bridge topology or for a modular multilevel converter cell having a full bridge topology.

According to an aspect, the modular multilevel converter unit of the arrangement comprising switches 101, 102, 111, 112, 121, 122, 131, 132, 141 according to an embodiment may further comprise a network interface for connecting the modular multilevel converter unit to a data network, in particular a global data network like the internet

In particular, the cells of the converter or the modular multilevel converter may be connected to an internal network, which is assigned to the converter or the cells as an intermediate converter control. The intranet may be connected to the internet.

In other words, the converter or its units communicate with the network (preferably in the field of the converter) and are dedicated to controlling the functions of the converter. It may serve as the middle control layer.

The data network may be a TCP/IP network such as the internet, and the modular multilevel converter unit or a controller within the converter unit may be operatively connected to a network interface for executing commands received from the data network via an internal network assigned to the converter as an intermediate converter control. The converter or converter unit may always be connected to the internet via an intermediate network, preferably the network is directly connected (on site) with the converter or converter unit.

The commands may comprise control commands for controlling the converter unit to perform tasks such as interrogating status data or interrogating operational data or measured sensor data. In this case, the converter unit or a controller of the converter unit (arranged inside or outside the unit) may be adapted to perform a task in response to a control command.

The command may include a status request. In response to the status request, or in the absence of a previous status request, the device/controller may be adapted to send status information to the network interface, which may then be adapted to send the status information over the network. The commands may include update commands that include update data. In this case, the device/controller may be adapted to initiate an update in response to the update command and use the updated data.

The data network may be an ethernet network using TCP/IP, such as a LAN, WAN or the internet. The data network may include distributed storage units, such as a cloud. Depending on the application, the cloud may be in the form of a public cloud, a private cloud, a hybrid cloud, or a community cloud.

In another embodiment, the transducer unit may further comprise a processing unit, which may be configured to convert the measured signal into a digital signal. The network interface may be configured to transceive digital signals/data between the unit or controller and a data network, wherein the digital signals/data include operational commands and/or information regarding the unit or network.

In another aspect, a method for shorting AC terminals of a modular multilevel converter cell may be disclosed. The method comprises the following steps: a defective switch is determined by a predetermined condition.

A defective semiconductor (transistor) may also mean that the control unit may not be able to switch the semiconductor in an "on" state or an "off state, but the switch may still be operable, and the control unit sending the control command to the switch may be defective. In this regard, not only the switch but also the control unit that outputs a control signal for the switch may be controlled. Therefore, the control unit may also be determined according to a predetermined condition. Such a condition may be, for example, a watchdog timer that may determine whether the output of the controller is activated according to a particular schedule.

In both cases, the switch state is no longer variable and the AC terminals should be shorted to achieve operability of the power converter.

According to another aspect of the application, the method may comprise activating a current path in the form of a semiconductor-based short-circuiting device (e.g. a crowbar) or activating a switch (semiconductor, in particular a transistor) in the current path (branch) such that a short-circuit loop between the AC terminals may be generated. To activate the switch, a control device may be used.

According to another aspect of the application, the method may further comprise activating the switch in such a way that: a switch that is considered healthy and operable according to predetermined conditions and that is arranged in parallel with a switch determined to be defective may be set to a permanent "on" state together with another switch that may be connected to the same DC potential as the failed switch. In another embodiment, a power converter including one or more cells according to embodiments of the present application may be disclosed. Furthermore, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet a further embodiment. It is contemplated that the description includes such modifications and variations.

While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.